It is known to prepare industrial and automotive grease formulations by mixing a thickener, for example a soap, into a suitable base oil. The oils used for this purpose tend to be mineral-derived base oils, typically of the same type as would normally be used in oil-based lubricants.
Depending on its intended use, the properties of a grease formulation need to be carefully tailored to meet applicable specifications and/or consumer demands. It needs to have a suitable consistency, for example. Ideally it should exhibit good mechanical stability and oil separation. Good oxidation stability and cold flow properties are also desirable, as is good anti-wear performance.
Often it can be difficult to achieve all of the desired properties in a typical mineral oil-based grease formulation. In such cases, one or more additives need to be included in the formulation so as to modify its performance. The inclusion of additives does, however, significantly increase the cost of producing the formulation. It would therefore be desirable to provide a grease formulation having certain desired properties, but with lower additive levels than are currently needed to achieve those properties.
Aside from the mineral-derived base oils, it is also now known to prepare base oils via a Fischer-Tropsch condensation process. This process is a reaction which converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g. 125 to 300° C., preferably 175 to 250° C.) and/or pressures (e.g. 5 to 100 bar, preferably 12 to 50 bar). Hydrogen:carbon monoxide ratios other than 2:1 may be employed if desired.
The Fischer-Tropsch process can be used to prepare a range of hydrocarbon fuels, including LPG, naphtha, kerosene and gas oil fractions. The heavier fractions can yield, following hydroprocessing and vacuum distillation, a series of base oils having different distillation properties and viscosities, which are useful as lubricating base oil stocks.
The higher molecular weight, so-called “bottoms” product that remains after recovering the lubricating base oil cuts from the vacuum column is usually recycled to a hydrocracking unit for conversion into lower molecular weight products, typically being considered unsuitable for use as a lubricating base oil itself. This product is often known as an “extra heavy” base oil cut.
Fischer-Tropsch derived base oils tend to have excellent low temperature properties, for example low pour points, and relatively good oxidation stability. They are also attractive because of the relatively simple process used to make them as compared to similar oils prepared from mineral crude sources. However they also have, as a result of the catalytic processes used to prepare them, relatively low polarity. This in turn gives them a relatively low affinity (solvency) for the high polarity thickeners (for example soaps) contained in grease formulations, and means that their inclusion in grease formulations would necessitate the use of relatively high thickener concentrations in order to achieve an appropriate consistency or stiffness (penetration). High thickener concentrations tend to be seen as undesirable due to the associated increased raw material costs. It is also generally believed that too high a thickener content in a grease formulation can lead to problems when pumping the formulation, particularly at lower temperatures; it is therefore thought to be desirable to seek to reduce rather than increase thickener concentrations.
It has now surprisingly been found that when a Fischer-Tropsch derived base oil is used in a grease formulation, with correspondingly increased levels of thickener, improvements in the properties and performance of the overall formulation can result, in particular in anti-wear performance. These improvements can in many cases outweigh the potential disadvantages of the higher thickener content.
Other relatively low polarity base oils have been used in grease formulations in the past, although not without their disadvantages. For example, synthetic polyalpha-olefins (PAOs) are occasionally used as a base for greases, but their high cost makes them suitable only for special applications. So-called “XHVI” (extra high viscosity index) oils, which are highly refined and chemically treated mineral oils, are also sometimes used in grease formulations, but these oils are only available with low viscosities which again limits their potential applications.
It would therefore be desirable to provide a grease formulation which could overcome or at least mitigate the above described problems, and ideally benefit from one or more improvements in overall performance.